The present invention relates to a method for controlling the current and voltage of a high voltage, direct current transmission line connected at one end to an alternating voltage system via a rectifier and at the other end to another alternating voltage system via an inverter, wherein a first circuit is employed to regulate one of the direct current and direct voltage output of the rectifier, a second circuit is employed to regulate one of the extinction angle of and current through the inverter, and a power regulator provides a signal representing a current reference magnitude to the first and second circuits. The invention also relates to a circuit arrangement for implementing the method.
A known regulating system for operating such a high voltage, direct current line is described in "BBC-Nachrichten" [BBC News], October/November 1970, pages 295-302.
In a direct current connection between two alternating current systems, the term HVDC-transmission (high voltage, direct current transmission) is used if a transmission line of considerable length is connected between the rectifier and inverter. If, however, no such line is present, one speaks of a back to back link.
The main purpose of direct current transmission is to transmit active power between the three-phase
Referring to the FIGURE, a three-phase transmission system 6 is connected by way of a transformer 7 to a full wave rectifier 2.0 comprising current valves 4 connected in a bridge circuit and being controllable by signals from pulse control 2.4. By way of a high voltage-direct current line 12 which includes inductances 10 and 11, rectifier 2.0 is in communication with an inverter 3.0 comprising current valves 3.8 connected in a bridge circuit and being controllable by signals from a pulse control device 3.4. Inverter 3.0 has its output connected to a further three-phase transmission system via a further transformer 8.
A power regulator 1, provided for regulating the power to be transmitted through line 12, receives a power reference parameter P.sub.REF, the direct voltage U.sub.d of line 12 obtained by means of a voltage detection device 2.5, and the current I.sub.d of line 12 obtained by means of a current transformer 2.6, and produces at its output a desired current magnitude I.sub.dREF which is fed to a rectifier current regulator 2.1 and an inverter current regulator 3.1. The desired current magnitude I.sub.dREF is reduced by a marginal current magnitude of .DELTA. I prior to being fed to current regulator 3.1. Current regulator 3.1 also receives the direct current I.sub.d in line 12 detected by way of a current transformer 3.6 as a regulating parameter. A switching member 3.3 selectively connects current regulator 3.1 or an extinction angle regulator 3.2 to pulse control device 3.4, depending on which one of the current regulator 3.1 and extinction angle regulator 3.2 sets the smaller control angle .alpha., where the control angle .alpha. is defined to be the electrical angle from the time when the anode voltage of a current valve becomes positive up to the firing instant. The extinction angle .gamma. is the electrical angle from the valve current extinction point up to the point that the commutation voltage becomes positive. The extinction angle regulator 3.2 receives an extinction angle reference parameter .gamma..sub.REF for the extinction angle .gamma. of inverter 3.0 and additional regulating parameters derived from the alternating voltage system 9 by means of a voltage transformer 3.5 and from the output of inverter 3.0 by a current transformer 3.7.
On the side of the circuit arrangement with rectifier 2.0, a switching member 2.3 selectively connects current regulator 2.1 or a direct voltage regulator 2.2 to the pulse control device 2.4 of rectifier 2.0, depending on which one of current regulator 2.1 and direct voltage regulator 2.2 sets the larger control angle .alpha.. Direct voltage regulator 2.2 receives a reference parameter U.sub.dREF, which in the prior art system corresponds to a reference parameter U.sub.dMAX (described below), to set the maximum permissible direct voltage magnitude in line 12. The regulating parameter U.sub.d is also provided to direct voltage regulator 2.2 from voltage detection device 2.5. Voltage regulator 2.2 is usually needed only to limit the direct voltage output of rectifier 2.0 so as to avoid voltage overload on components. Its reference input U.sub.dMAX generally corresponds to 105 to 115% of the rated direct voltage in line 12.
During normal operation, switching member 2.3 operates to connect current regulator 2.1 to pulse control device 2.4 for regulating the firing of the valves 2.8 in rectifier 2.0. Since current regulator 2.1 sets a current in line 12 which is larger by .DELTA.I than what is set by current regulator 3.1, current regulator 3.1 increases the countervoltage of inverter 3.0 until the output of current regulator 3.1 is replaced by the output of extinction angle regulator 3.2 which determines the highest inverter voltage.
If rectifier 2.0 does not develop sufficient current because, for example, voltage U.sub.L in alternating voltage system 6 is too low, current regulator 3.1 will reduce the control angle .alpha. of valves 3.8 and thus reduce the counter-voltage of inverter 3.0 until a current flows in line 12 which corresponds to the desired current parameter I.sub.dREF.
The magnitude of the three-phase voltage U.sub.L of three-phase system 6 can be influenced by reactive power obtained at the bus bar. Inductive reactive power here produces a reduction in voltage.
Both rectifier 2.0 and inverter 3.0 of the direct current line take inductive reactive power from their a.c. systems which is proportional to the direct current flowing in line 12 and approximately proportional to the sine of the control angle .alpha.. The control angle .alpha. of the rectifier 2.0 can be influenced by way of the direct voltage regulator 2.2, wherein a maximum voltage U.sub.d occurs with .alpha.=0. With increasing .alpha. the voltage U.sub.d becomes lower in that U.sub.d is proportional to cos.alpha..
Direct current links are generally constructed in such a manner that during rated operation the reactive power they require is supplied by capacitors (see, for example, German Offenlegungsschrift [laid-open patent application] No. 2,228,042). If the system operates under partial load, too much capacitive power is present and the three-phase voltage may become too high. In this case, it would be desirable to operate the direct current link with a low direct voltage. Several proposals have already been made to achieve this, most of them effecting operation with a constant power factor (see, for example, IEEE Trans. PAS 89 No. 6 (1970) pages 1120-1126, Kanngiesser, Lips).
The present invention is based on the realization that in direct current links the transmission of the desired active power takes priority. Since this active power has its rated value at full direct voltage and rated current, it is possible to influence the reactive power under partial load. With a partial load and unlimited direct voltage, the power regulator will set a smaller direct current.